Re: [PATCH v7 1/7] KVM: CPUID: Fix IA32_XSS support in CPUID(0xd,i) enumeration

Yang Weijiang
2019-10-18T01:25:14Z

On Thu, Oct 17, 2019 at 12:46:22PM -0700, Sean Christopherson wrote:
> On Wed, Oct 02, 2019 at 10:26:10AM -0700, Jim Mattson wrote:
> > On Thu, Sep 26, 2019 at 7:17 PM Yang Weijiang <weijiang.yang@intel.com> wrote:
> > > @@ -414,6 +419,50 @@ static inline void do_cpuid_7_mask(struct kvm_cpuid_entry2 *entry, int index)
> > > }
> > > }
> > >
> > > +static inline void do_cpuid_0xd_mask(struct kvm_cpuid_entry2 *entry, int index)
> > > +{
> > > + unsigned int f_xsaves = kvm_x86_ops->xsaves_supported() ? F(XSAVES) : 0;
> >
> > Does Intel have CPUs that support XSAVES but don't support the "enable
> > XSAVES/XRSTORS" VM-execution control?
>
> I doubt it.
>
> > If so, what is the behavior of XSAVESXRSTORS on those CPUs in VMX
> > non-root mode?
>
> #UD. If not, the CPU would be in violation of the SDM:
>
> If the "enable XSAVES/XRSTORS" VM-execution control is 0, XRSTORS causes
> an invalid-opcode exception (#UD).
>
> > If not, why is this conditional F(XSAVES) here?
>
> Because it's technically legal for the control to not be supported even
> if the host doesn't have support.
>
> > > + /* cpuid 0xD.1.eax */
> > > + const u32 kvm_cpuid_D_1_eax_x86_features =
> > > + F(XSAVEOPT) | F(XSAVEC) | F(XGETBV1) | f_xsaves;
> > > + u64 u_supported = kvm_supported_xcr0();
> > > + u64 s_supported = kvm_supported_xss();
> > > + u64 supported;
> > > +
> > > + switch (index) {
> > > + case 0:
> > > + entry->eax &= u_supported;
> > > + entry->ebx = xstate_required_size(u_supported, false);
> >
> > EBX could actually be zero, couldn't it? Since this output is
> > context-dependent, I'm not sure how to interpret it when returned from
> > KVM_GET_SUPPORTED_CPUID.
>
> *sigh*. It took me something like ten read throughs to understand what
> you're saying.
>
> Yes, it could be zero, though that ship may have sailed since the previous
> code reported a non-zero value. Whatever is done, KVM should be consistent
> for all indices, i.e. either report zero or the max size.
>
Thanks Seans! So I will add the check *if (!supported)* back in next
version.

> > > + entry->ecx = entry->ebx;
> > > + entry->edx = 0;
> >
> > Shouldn't this be: entry->edx &= u_supported >> 32?
>
> Probably. The confusion likely stems from this wording in the SDM, where
> it states the per-bit behavior and then also says all bits are reserved.
> I think it makes sense to do as Jim suggested, and defer the reserved bit
> handling to kvm_supported_{xcr0,xss}().
>
> Bit 31 - 00: Reports the supported bits of the upper 32 bits of XCR0.
> XCR0[n+32] can be set to 1 only if EDX[n] is 1.
> Bits 31 - 00: Reserved
>
> > > + break;
> > > + case 1:
> > > + supported = u_supported | s_supported;
> > > + entry->eax &= kvm_cpuid_D_1_eax_x86_features;
> > > + cpuid_mask(&entry->eax, CPUID_D_1_EAX);
> > > + entry->ebx = 0;
> > > + entry->edx = 0;
> >
> > Shouldn't this be: entry->edx &= s_supported >> 32?
>
> Same as above.
>
Yes, I followed Jim's comments.

> > > + entry->ecx &= s_supported;
> > > + if (entry->eax & (F(XSAVES) | F(XSAVEC)))
> > > + entry->ebx = xstate_required_size(supported, true);
> >
> > As above, can't EBX just be zero, since it's context-dependent? What
> > is the context when processing KVM_GET_SUPPORTED_CPUID? And why do we
> > only fill this in when XSAVES or XSAVEC is supported?
> >
> > > + break;
> > > + default:
> > > + supported = (entry->ecx & 1) ? s_supported : u_supported;
> > > + if (!(supported & ((u64)1 << index))) {
> >
> > Nit: 1ULL << index.
>
> Even better: BIT_ULL(index)
>
> > > + entry->eax = 0;
> > > + entry->ebx = 0;
> > > + entry->ecx = 0;
> > > + entry->edx = 0;
> > > + return;
> > > + }
> > > + if (entry->ecx)
> > > + entry->ebx = 0;
> >
> > This seems to back up my claims above regarding the EBX output for
> > cases 0 and 1, but aside from those subleaves, is this correct? For
> > subleaves > 1, ECX bit 1 can be set for extended state components that
> > need to be cache-line aligned. Such components could map to a valid
> > bit in XCR0 and have a non-zero offset from the beginning of the
> > non-compacted XSAVE area.
> >
> > > + entry->edx = 0;
> >
> > This seems too aggressive. See my comments above regarding EDX outputs
> > for cases 0 and 1.
> >
Sean, I don't know how to deal with entry->edx here as SDM says it's
reserved for valid subleaf.

> > > + break;
> > > + }
> > > +}